1. Field of the Invention
This invention relates to a focus condition detecting system of the TTL (through the lens) phase difference detecting type and interchangeable lenses for the system, particularly of the cata-dioptric type.
2. Description of the Prior Art
A focus condition detecting device of the TTL phase difference detecting type is widely employed in automatic focusing systems of single lens reflex cameras. In a focus condition detecting system of the type mentioned, a focus condition is determined in accordance with a distance between first and second images of light formed from light from an object which has passed first and second portions of a plane of an exit pupil of an interchangeable lens. In the focus condition detecting device, the position and size of an AF (automatic focusing) pupil which is to be passed by an AF beam of light to be used for detection of a focus condition depend upon designing of a camera body. Where the AF pupil is within an exit pupil of an interchangeable lens, no eclipse will appear in an AF beam of light and no trouble will occur in a focus condition detecting operation. However, the positions or sizes of exit pupils of interchangeable lenses are not constant, and in some cases, an AF pupil may be displaced from an exit pupil so that an eclipse may appear in an AF beam of light. In such an instance, there is a problem that an error appears in detection of a focus condition or in some cases, detection of a focus condition is disabled.
Thus, it has been proposed to prepare a plurality of different types of pairs of focus condition detecting optical sensors and selectively use one of the pairs of focus condition detecting optical sensors in accordance with a fully open aperture value of a lens used (Japanese Patent Publication No. 62-6206). However, since the prior art arrangement does not involve accurate comparison in dimensional relationship or positional relationship between an exit pupil of an interchangeable lens and an AF pupil of a camera body, presence or absence of an eclipse in an AF beam of light cannot be discriminated with accuracy. Besides, it is not at all taken into consideration that particularly an exit pupil may be limited also on the inside thereof as in a cata-dioptric lens.
A cata-dioptric lens is very advantageous in that it is very inexpensive and compact. It is thus desired that detection of a focus condition or automatic focusing is enabled even if a cata-dioptric lens is mounted on a body of a camera. While cata-dioptric lenses of the type which can be mounted on a single lens reflex camera having an AF function of the TTL phase difference detecting type are conventionally on the market, such a cata-dioptric lens is not designed such that, where it is mounted on a body of a camera, information of the lens is forwarded to the camera body. Accordingly, the AF function of the camera body will not operate.
In the arrangement disclosed in Japanese Patent Publication No. 62-6206, whether detection of a focus condition is possible or impossible is judged depending upon a fully open aperture value of a lens used as described above. However, where the lens used is a cata-dioptric lens, a beam of light will be limited also on the inside of an exit pupil of the lens as described above. Accordingly, it is a problem that whether detection of a focus condition is possible or impossible cannot be judged correctly only from data of a fully open aperture value.
Meanwhile, as interchangeable lenses for single lens reflex cameras having AE (automatic exposure) and AF functions of the TTL type, a large number of lenses have been developed wherein data for automatic exposure such as a fully open aperture value, a maximum aperture value and a focal length and data for automatic focusing such as a lens feeding amount converting coefficient are stored fixedly such that they may be accessed from a camera body. However, no lens has been proposed wherein data of an exit pupil is fixedly stored together with a fully open aperture value. This is because such data of an exit pupil can be found in most cases from data of a free open aperture value.
However, if it is intended to enrich a camera system, this will increase the necessity of data regarding radii or diameters of an exit pupil of a lens. For example:
(1) When it is intended, in a camera having an AF function of the TTL phase difference detecting type, to expand an area over which a focus condition is to be detected, it is necessary, in order to judge whether or not detection of a focus condition is possible for such an expanded area, to strictly judge whether a beam of light with which a focus condition is to be detected belongs to a plane of an exit pupil of an interchangeable lens. For such judgement, however, strict data of a radius or radii of an exit pupil of the lens are necessitated and will not be satisfied by data of a fully open aperture value.
(2) When it is intended to enrich interchangeable lenses, it may be necessary to add, for example, a cata-dioptric lens, to ordinary lenses of the dioptric type. Such a cata-dioptric lens has an exit pupil having a quite different shape from those of ordinary lenses of the dioptric type. Accordingly, in the case of a cata-dioptric lens, data of a fully open aperture value will not be substituted for data of an exit pupil.